Optical coatings, which include one or more films of dielectric or metallic materials, are widely used in both research and commercial applications from mirrors to eyeglasses to photography lenses. Many conventional dielectric coatings rely on Fabry-Perot-type interference, involving multiple optical passes through transparent layers with wavelength-scale thickness to achieve effects such as anti-reflection, high-reflection, and dichroism. Highly absorbing dielectrics are generally not used because wavelength-scale propagation through such media may limit coherent optical effects.
Conventional optical coatings generally comprise some combination of thin metallic films serving as partial and full reflectors, and wavelength-scale dielectric films which rely on Fabry-Perot-type or “thin film” interference—the same effect that is responsible for colorful patterns on oil films and soap bubbles. Common examples include anti-reflection (AR) and high-reflection (HR) coatings, which are often made by stacking layers of dielectrics with quarter-wave thickness (λ/4n, where n is an index of refraction of the material). These interference effects rely on multi-pass light circulation within the optical cavities formed by the films, and are typically very sensitive to the angle of incidence. By engineering multi-layer dielectric stacks, more complex and robust devices, such as omni-directional reflectors may be created—e.g., optical coatings are primarily designed and optimized by computer software. However, such a design may be time and cost intensive both to design and to fabricate.